Output transformer ambiguity

[Steve Sousa]

"Meat Plow" <mhywatt@yahoo.com> wrote in message
newsan.2010.11.23.20.18.12@lmao.lol.lol...

On Tue, 23 Nov 2010 18:21:50 +0000, Steve Sousa wrote:

That's a heck of a difference in DC resistance. If you drive both with a
sig generator and look at the secondaries on a scope is there a big
difference in the signal amplitude between the two? If not I would be
inclined to replace the tube and take some voltages while the circuit is
live while monitoring the plate currents. You can look here for
operational voltages and currents in different classes of operation:

http://www.nj7p.org/Tube4.php?tube=6gu8

hello:

there was about half division difference on the scope. I was trying to avoid
testing with the valve because it's expensive and "rare" i don't want to
take much risks....

 

[N_Cook]

Steve Sousa <etsteve@yahoo.com> wrote in message
news:4cee6eaa$0$61360$892e0abb@auth.newsreader.octanews.com...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:icijdo$d1a$1@news.eternal-september.org...
If the 900 ohm one was shot then it implies some shorted turns that is
unlikely to change with heat and/or vibration



Hello:

the amp is a grundig como e/st, you can see a picture and the schematic at
http://meuble.radio.free.fr/stereo_console/grundig/index.html

i haven't removed the transformers because they are soldered to the
chassis!
(the metal bracket is soldered to the chassis *and* the pcb)

Do the 2 in one output valves mean the amp is compact size?
I know an intermittent break in low impedance side of output Tx can lead to
valve failure but does intermittant high imp side harm the drive valves?
The Tx numbers matching on that schematic confirms they should be to within
about 5 percent the same measurements

 

[Jeff Layman]

On 25/11/2010 14:09, Steve Sousa wrote:

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8l2pujF1hkU1@mid.individual.net...


** Assuming the transformers are the same size and do the same job,
the one with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got
45,6 and 46,0


** Do not use a comma for the decimal point - it is way too confusing.



..... Phil


Hello

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

Best Regards

Are these of any use:
http://www.vvttransformers.co.uk/vtp135200.htm
http://www.vvttransformers.co.uk/vtp206300.htm

--

Jeff

 

[Phil Allison]

"Steve Sousa"

"Phil Allison"

** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got 45,6
and 46,0


** Do not use a comma for the decimal point - it is way too confusing.


Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

** No.

The ratio between rated primary Z and winding resistance is never less than
5:1 and is typically 10:1 or more.



..... Phil

 

[David Nebenzahl]

On 11/25/2010 12:01 PM Phil Allison spake thus:

"Steve Sousa" wrote:

"Phil Allison" wrote:

** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

** No.

The ratio between rated primary Z and winding resistance is never less than
5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct?
Two main possibilities here: open windings, or shorted windings (or I
suppose a combination). Obviously not an open winding, so probably
shorted ones.

Shorted windings could be within the primary or secondary, or between.
Let's look at the simpler case, shorted within the primary, which should
be 4K ohms.

It seems most likely that any short would be between adjacent layers of
windings. Not likely that there'd be a short from close to one end to
close to the other end. Which should limit the effect of the short. So
if the 4K ohm primary is now 900 ohms, there's something else going on.

But what? Could there be a short from the winding to the iron core?

Has the O.P. checked for a short between primary and secondary? How
about shorts to the core/frame of the xfmr?


--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

 

[Phil Allison]

"David Nebenzahl = Idiot "

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm
too?

** No.

The ratio between rated primary Z and winding resistance is never less
than 5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct? Two
main possibilities here: open windings, or shorted windings (or I suppose
a combination). Obviously not an open winding, so probably shorted ones.

** Shame about the third and most likely scenario - one that makes the
primary R go way high.

Think " vertigris " .....



...... Phil

 

[N_Cook]

Steve Sousa <etsteve@yahoo.com> wrote in message
news:4cee6eaa$0$61360$892e0abb@auth.newsreader.octanews.com...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:icijdo$d1a$1@news.eternal-september.org...
If the 900 ohm one was shot then it implies some shorted turns that is
unlikely to change with heat and/or vibration



Hello:

the amp is a grundig como e/st, you can see a picture and the schematic at
http://meuble.radio.free.fr/stereo_console/grundig/index.html

i haven't removed the transformers because they are soldered to the
chassis!
(the metal bracket is soldered to the chassis *and* the pcb)

What is the result of hot-air heating and knocking the transformers with
copper/hide hammer or simulating with hammer and wood block while monitoring
the ohms?
Certainly don't power up with new valve without using a variac and probably
filament bulb telltales(acting as fuse if serious problem) in place , if you
cannot establish which Tx is wonky.

 

[N_Cook]

I would go for shorted turns on the "900" ohm one especially if heating and
knocking fails to show any variation. Shorted turns tend to be "welded"
together . I only have test data for the EL80 but 25mA per pentode and
looking at some Premier Tx data for range of use 7 watt to 60W, DC of HT
side ranging from 1500 to 16000 ohm so 4000 ohm seems more likely.
What is the physical dimensions of your ones?

 

[David Nebenzahl]

On 11/25/2010 5:22 PM Phil Allison spake thus:

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm
too?

** No.

The ratio between rated primary Z and winding resistance is never less
than 5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct? Two
main possibilities here: open windings, or shorted windings (or I suppose
a combination). Obviously not an open winding, so probably shorted ones.

** Shame about the third and most likely scenario - one that makes the
primary R go way high.

Think " vertigris " .....

You mean *verdigris*.

So that could happen if the insulation got nicked and moisture gets in,
corroding the wire. How does that work? Narrow the wire and increase the
resistance (eventually causing an open)?


--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

 

[Phil Allison]

"David Nebenzahl"
Phil Allison

** Shame about the third and most likely scenario - one that makes the
primary R go way high.

Think " vertigris " .....

You mean *verdigris*.

So that could happen if the insulation got nicked and moisture gets in,
corroding the wire. How does that work? Narrow the wire and increase the
resistance (eventually causing an open)?

** Can happen any number of ways including at the termination points of the
enamelled wire where it is soldered to a flying lead. Solder flux is then
the trigger and moisture from the air can get to it.

The high resistance is because for some small distance, conduction is via
the verdigris.



..... Phil